Taking in consideration of environmental factors, a replacement of a CCFL (Cold Cathode Fluorescent Lamp) with LEDs in a backlight unit for use in a liquid crystal display device is now proceeding. CCFL-type backlight units have employed, for dimming itself, a way to apply a signal of sinusoidal wave to a CCFL. In contrast, LED-type back light units mainly employ, for dimming itself, a PWM (Pulse Width Modulation) method wherein a signal of rectangular wave is applied to LEDs.
FIG. 1 illustrates a structural example of a general light-emitting element driving circuit which includes a light-emitting element drive unit and light-emitting elements. This light-emitting element driving circuit employs LEDs as the light-emitting elements. Power supply voltage is externally applied to the light-emitting element driving circuit. In the light-emitting element drive unit, the voltage is increased by coil 2 and FET (Field Effect Transistor) 3. This structural example employs, as a switching signal to be applied to a gate of FET 3, the signal outputted by LED driver IC (Integrated Circuit) 7. The increased anode voltage is applied to an anode-side part of LED arrays 6 through schottky-barrier diode 4 for rectification. When the voltage is applied to LED arrays 6, the light-emitting element driving circuit choose whether to apply an electric current to LEDs or not in accordance with switching transistor 7a, and determines whether to turn the LEDs on or off. To control an ON/OFF state of the LEDs, a PWM signal for dimming is externally inputted to the circuit. To increase or decrease the luminance of a backlight unit, the ON and OFF periods of the LEDs are controlled to obtain a desired brightness. In this structure, capacitor 1 is a smoothing capacitor at the input-terminal side and capacitor 5 is a smoothing capacitor at the output-terminal side.
As an example of such an LED drive circuit, Japanese Unexamined Patent Application Publication (JP-A) No. 2012-15369 discloses an LED control device for driving plural LEDs, having the following structure. The LED control device is configured to turn each of LED groups on or off separately, where the LED groups are connected to a plurality of constant-current output circuits. The LED control device includes on LED drive section, a power-supply control section and a phase-difference control section. The LED drive section is configured to control whether to supply a current to each of the LED groups by the corresponding constant-current output circuit separately, in accordance with a PWM signal inputted corresponding to each of the LED groups. When at least one of the LED groups is turned on, the power-supply control section controls a power supplied from a power-supply device to which the LED groups are connected in parallel, by a first voltage control mode in which a voltage at a cathode-side end of the at least one of the LED groups to be turned on is kept at a first predetermined voltage value. When all the LED groups are turned off, the power-supply control section controls the power by a second voltage control mode in which a voltage at an anode-side end of the LED groups is kept at a second predetermined voltage value. The phase-difference control section gives a phase difference of 2π/n (where n is the number of LED groups) to each of the PWM signals which correspond respectively to the LED groups and are inputted to the LED drive section. FIG. 17A to 17C illustrate an example of operations of the LED device circuit.
As another example, JP-A No. 2011-13866 discloses an LED backlight drive circuit having the following structure. The LED backlight drive circuit includes a switched-mode DC/DC convertor, a LED driver IC and a load circuit. The switched-mode DC/DC convertor includes a smoothing capacitor around the output terminal thereof and is configured to supply a drive current to an LED backlight wherein one or more light-source arrays each including plural LEDs connected in series, are connected in parallel. The LED driver IC is configured to turn on or off the drive current which comes from the DC/DC converter and passes through the LED backlight in accordance with a PWM control signal. The load circuit is connected to the output terminal of the DC/DC converter, wherein during a period the drive circuit of the LED backlight is turned off, a current which is equivalent to the drive current enters from the DC/DC converter to the load circuit. The load circuit includes a load resistance connected to the output terminal of the DC/DC converter in parallel with the input terminal of the LED backlight, a switching element configured to turn on/off the flow of a current to the load resistance, and a switching control circuit configured to perform on-off control of the switching element to be synchronized with a PWM control signal. FIG. 18 illustrates an example of operations of the LED backlight drive circuit.
As another example, JP-A No. 2011-242570 discloses an LED video display device including plural LED display units arrayed to form an LED screen, where in each of LED display units a large number of LEDs are mounted in a matrix shape. The LED video display device further includes a constant-voltage supplying section, a screen controller and a drive circuit for the LED display units. The constant-voltage supplying section supplies power for respective LEDs mounted on the plural LED display units. The screen controller is configured to perform a light-emission control of respective LEDs by sending PWM instructions to each of the plural LED display units and adjusting a current passing each of the plural LEDs. The drive circuit for the LED display units is composed of a switching circuit and a switch control section. The switching circuit is composed of switches connected to the LEDs in series, respectively. The switch control section is configured to perform an ON/OFF control on the LEDs in response to the PWM instructions. The switch control section is configured to change the duration of a pulse for turning each LED on based on the PWM instructions and adjust the peak-time width of the current which is supplied by the constant-voltage supplying section and passes the LED display units. FIGS. 19A to 19C illustrate examples of operations of the LED video display device. FIG. 19A illustrates an example that one 20-millisecond light emitting period includes one 10-millisecond pulse for turning each LED on, FIG. 19B illustrates an example that one 20-millisecond light emitting period includes two 5-millisecond pulses for turning each LED on, and FIG. 19C illustrates an example that one 20-millisecond light emitting period includes five 2-millisecond pulses for turning each LED on.
Regarding backlight units for use in liquid crystal display devices, a replacement of a CCFL with LEDs is now proceeding. However, phenomenon which did not cause large troubles in CCFL-type backlight units can make large troubles in LED-type backlight units as described below.
On dimming a backlight unit by using a PWM signal, there are caused repeats of a steep increase and decrease of a current which passes a load connected to the output end of the light-emitting element drive circuit, which makes ripples in both of the output current and the output voltage of a DC/DC converter in a light-emitting element driving circuit.
Many of light-emitting element driving circuits employ a ceramic capacitor which is made of a piezoelectric material. Therefore, when ripples appear in a voltage to be applied to the capacitor, the capacitor vibrates and makes acoustic noise, which is a property of piezoelectric materials. Especially, since the frequency of a PWM signal to be used for the dimming falls in the human audible frequency range in many cases, the acoustic noise coming from the capacitor can cause a large problem.
In view the problem, many methods to shift the frequency of the PWM signal away from the human audible frequency range can be considered. However, if the frequency of the PWM signal decreases, a screen flicker can be observed by human eyes on the dimming, which is not preferable. On the other hand, if the frequency of the PWM signal increases, a circuit which can work at high speed is required and such a circuit causes an increase of cost, which is not preferable, too.
Further, the structure of JP-A No. 2012-15369 can make a variation of a current smaller in comparison with the case that all the LED arrays are driven at the same time, and can restrict the acoustic noise. However, since the frequency and the cycle of the PWM signal for dimming LEDs are constant and an influence of the component of a certain frequency strongly becomes large, acoustic noise is perceived by users.
In the structure of JP-A No. 2011-138666, a dummy signal generated based on a PWM signal is applied to a dummy load connected in parallel with the LEDs during a period that LEDs are turned off by a dimming operation using a PWM signal, so as to reduce the number of times of fluctuations of a voltage on a DC/DC output section to be applied to an anode. This structure restricts overall fluctuations of the current and shifts the frequency of the acoustic noise to be lower than the human audible frequency range, which can restrict the acoustic noise. However, this structure increases the area of the t-emitting element driving circuit because of the dummy load, and hardly restricts its electricity consumption on dimming the LEDs. Further, in order to keep the voltage of the DC/DC output section in this structure, it is required that a current which is equivalent to the current passing the LEDs is supplied to the DC/DC output section. Therefore, the electricity consumption increases with corresponding to an increase of the number of LEDs, and the area of the dummy load also increases in order to restrict a heat generation. As for the acoustic noise, high-frequency noise having frequencies each being an odd multiple of the original frequency are mainly perceived by users, which means that acoustic noise in the human audible frequency range is generated finally.
The structure of JP-A No. 2011-242570 is configured to detect the loudness of the acoustic noise and selects the frequency of the PWM signal according to the detected loudness. However, the structure uses only given frequencies and a selected frequency from among the given frequencies does not always work on the acoustic noise generated depending on the shape of the light-emitting element driving circuit and the way to arrange the light-emitting element driving circuit.
The present invention seeks to solve the above-described problems.